The enzyme p-diphenol oxidase (laccase; E.C. 1.10.32) acts to convert a yellow precursor to the green pigment characteristic of the asexual spores (conidia) of the fungus Aspergillus nidulans (Clutterbuck, 1972). This enzyme activity is present at very low levels in non-conidiating cultures and is elevated to high levels during conidiation, that is, it is developmentally regulated. It should prove to be especially useful for the investigation of gene control during fungal development because 1) its physiological role is known, 2) it is not essential for normal growth and reproduction, 3) it is produced in significant quantities, and 4) it is accessible to mutational as well as biochemical analysis. At the present time, it is not known whether the increase in enzyme activity during development is due to an increase in the transcriptional activity of the structural gene, information which is essential for future studies. I therefore propose to characterize the activity of the structural gene by determining the timing of enzyme protein and messenger RNA accumulation and synthesis during conidiation. During the course of these experiments, recombinant DNA molecules will be produced and propagated which contain the p-diphenol oxidase structural gene. These clones will be used to investigate the organization of the laccase gene and the transcription and processing of laccase mRNA. It is anticipated that the results of these experiments will provide insights into the mechanisms controlling gene activity in fungi and higher eucaryotes and thereby contribute to our understanding of normal and abnormal cell growth and differentiation and of reproduction in fungi which cause disease in man.